Effective dose (E) is the only comparatively simple dose quantity that is related to health detriment for stochastic effects from exposure to ionising radiation. As such, E has found wide application for medical exposures, as it allows comparisons with doses from different examinations and other sources. E is derived from the weighted sum of doses to tissues known to be sensitive to radiation from epidemiological studies and contains inherent approximations. Thus it is not a scientific quantity, but a practical one that the International Commission on Radiological Protection (ICRP) has created for use in the calculation of reference doses for protection purposes. In the application of E to medical exposures, there has been a tendency to attribute a greater accuracy to values of E than is justified by its derivation. Recognising that E is strictly not subject to uncertainties, an analysis has been undertaken of potential uncertainties in E for different nuclear medicine examinations to enable users to judge its reliability as a comparator of relative risk. Assessments have been based on the considered accuracy of the component parts and indicate that the uncertainties in the values of E as a relative indicator of harm for nuclear medicine procedures for a reference patient are about ± 50%. These are larger than those for radiology procedures, because of the tendency for doses to single organs, especially the bladder, to form a substantial part of E for some procedures. Revision of the tissue weighting factors in 2007 produced a 10% decrease in the mean value of E for nuclear medicine examinations. Estimations of cancer risk based on E for an individual could vary by one or two orders of magnitude. E fulfils an important role as a health-related dose quantity that can be used in justification of nuclear medicine examinations, but physicians should be aware of its limitations. General terminology should be used in conveying risks to patients and medical professionals.